1. Prior Art: X-ray Lithography
Current commercial lithography techniques utilize optical and ultraviolet light to expose photoresist through a mask which has the circuit pattern imprinted on it. Present geometries of the circuit elements are limited by the wavelength of the radiation to greater than 0.5 microns in size. To produce smaller geometries, shorter wavelengths are needed. The use of soft X-rays for the exposure of photoresist will produce geometries of less than 0.25 microns; lines as small as 0.16 microns have already been produced.
Four X-ray sources have been considered for lithography. These are the conventional X-ray tube (or electron impact sources), synchrotron storage ring, laser plasma sources and transition radiators. To date synchrotron and plasma sources have been the most widely discussed and used. Electron-impact sources have intrinsically low X-ray power, requiring 20 minutes or more for small area exposures, and are uncollimated, requiring stringent mask-to-wafer spacing.
Synchrotron radiation has received the most interest as a commercial source for the high-volume production of integrated circuits, where laser-plasma sources have been touted as a relatively inexpensive source of soft X-rays for low-volume integrated-circuit production. The properties of synchrotron radiation which make it useful for X-ray lithography are its high intensity and collimation. See for example E. Spiller and R. Feder, "X-Ray Lithography," Topics in Applied Physics, vol. 22 (ed. H. J. Queisser; Springer Verlag, Berlin, Heidelberg, New York, 1977.)
Unfortunately, synchrotron radiation requires massive machines costing $25 million for the storage ring and $2.5 million per lithographic station for a total of 16 to 24 stations. A 16 station synchrotron based lithography system is projected to have a per station cost of $4M to $5M and a total system cost of $64M to $80M. This exorbitant initial cost has prevented most U.S. companies from entering into X-ray lithography. Thus there is considerable interest in a relatively inexpensive X-ray lithography source which can have an initial cost of 4 to 6 million dollars and have one or several stations with the same cost of 4 to 6 million dollars per station. Laser-plasma sources are a potential single-station source of X rays in this price range. Hampshire Laboratories is producing an experimental system but the system has demonstrated only 2 milliwatts of X-ray power.
Laser-plasma sources have the intrinsic technical problems of low power and lack of collimation. Low X-ray power limits this source to an experimental research tool or, to low volume integrated-circuit production. Lack of collimation requires it to be extremely close to the mask and wafer (10 to 20 cm) to maintain flux density, which then requires the spacing between the mask and wafer to not more than .+-.0.5 .mu.m variation to minimize shadowing and blurring of the circuit image. This required tolerance of .+-.0.5 .mu.m is difficult to achieve and may be prohibitive. The synchrotron source requires a maximum variation of only .+-.5 .mu.m. A new X-ray source is needed with the synchrotron's excellent technical characteristics and laser-plasma's moderate cost.
2. Prior Art: Transition Radiation as an X-ray Lithography Source
In the prior art, transition radiation has been considered as an alternative source of soft X rays by M. A. Piestrup, J. O. Kephart, H. Park, R. K. Klein, R. H. Pantell, P. J. Ebert, M. J. Moran, B. A. Dahling, and B. L. Berman, "Measurement of transition radiation from medium-energy electrons," Phys. Rev. A vol. 32, pp. 917-927, Aug. 1985. and M. A. Piestrup, M. J. Moran, B. L. Berman, P. Pianetta, D. Seligson, "Transition radiation as an X-ray source for lithography," SPIE vol. 773, Electron-Beam, X-ray, and Ion-Beam Lithographies, pp. 37-44, 1987.
Transition radiation has a number of advantages which it shares with synchrotron emitters. Both are high brightness, collimated source. The high degree of radiation parallelism (collimation) decreases geometrical distortions such as run-out and blurring of the circuit elements in a lithograph (See FIG. 1). Unique advantages of transition radiation are its relative low cost, moderate vacuum requirements and excellent spectral characteristics.
One advantage synchrotron radiation now has over transition radiation is that of total X-ray power. Although the X-ray production by transition radiation is at least three orders of magnitude brighter on a per electron basis, storage rings have a higher average beam current, and this advantage is lost.
The power density required for lithography is between 10 to 100 mW/cm.sup.2. Larger values than 100 mW/cm.sup.2 will result in excessive heating and possible damage to the X-ray mask. Lower values than 10 mW/cm.sup.2 will result in prohibitively long exposure times.
Exposure times depend upon resist sensitivity that can vary between 10 mj/cm.sup.2 to 1000 mj/cm.sup.2. Higher resolution resist usually requires higher energy deposition. For example, polymethylmethacrylate (PGMA) photoresist with 0.25 .mu.m resolution or better requires 230 mj/cm.sup.2. This experimental resist would require 15 mW/cm.sup.2 for 15 second exposures.
In the prior art, transition radiators have produced only fluxes of 0.7 mW (see M. A. Piestrup, M. J. Moran, B. L. Berman, P. Pianetta, D. Seligson, "Transition radiation as an X-ray source for lithography," SPIE vol. 773, Electron-Beam, X-ray, and Ion-Beam Lithographies, pp. 37-44, 1987). The Lawrence Livermore National Laboratory's (LLNL) electron-position accelerator was utilized to produced a 104-MeV (million electron volts), 44-.mu.A (amperes) electron beam which penetrated a stack of fifteen 1.5-.mu.m-thick aluminum foils generating only 0.7 mW of soft X rays at a peak photon energy of 1.4 keV (kilo-electron volts) with an approximate bandwidth of 100%. Exposure times for the mask and wafer (here in after called the mask/wafer) were long due to the large area of the X-ray beam and the large distance between the radiator and the mask/wafer (6 m). Since the X ray annulus diverges at 1/.gamma., after 6 m the area of the annulus is much larger than 6 to 12 cm.sup.2 for electron beam energies of 150 MeV and larger. Fluxes of 60 mW exposing area of 6 cm.sup.2 or less are needed for a 10 mW/cm.sup.2. The conical X-ray pattern emitted from the transition radiator used in this experiment was not uniform and exposed only an annular ring on the photoresist. Thus methods for making the radiation pattern uniform and increasing the X-ray power are needed.
In the prior art it was believed that transition radiation was not adequate for high throughout (high production) of integrated-circuit chips since the exposed area was small and the total power of the transition radiator was small. Wafers of 4 to 6 inches in diameter are used in optical lithography, and it was assumed that transition radiation had to expose equivalent size areas: the total X-ray flux had to cover the entire area of the 4 to 6 inch wafers and still maintain a minimum of 10 mW/cm.sup.2 average power. Since the total from the transition radiator is limited, the number of chips per hour that this source could produce would be small. Thus transition radiation has not been seen as a competitor for X-ray lithography.
In the prior art Holman and others assumed that extremely high currents were needed to generate adequate soft X-ray flux from transition radiators needed for X-ray lithography. Holman suggests a 1 mA, 100 MeV electron beam. This is 100 kW (kilowatts) of power, a difficult electron-beam power to achieve. In addition, the foils will melt as such a high current and the radiation hazard generated by such a high-current electron beam is appreciable. As Holman, himself says, "The assumption of beam uniformity is optimistic, as is the 1 mA current." Richard Holman, Intel Corporation, "X-ray lithography using broadband sources" (1986).